Oriented plastic film, particularly oriented polyester film, more particularly biaxially oriented film composed of polyethylene terephthalate (PET), is widely used as a base for drafting film, photographic film and reprographic film, as well as for packaging and labelling applications. Although equally applicable to plastic film in general, the description will be directed mainly towards PET for convenience.
Because PET film is hydrophobic and is not readily receptive to coating or printing, in most applications where the film is to serve as a base or support for other coatings, it must be first coated on one or both sides with a primary coating which adheres to the film and is receptive as well to other coatings applied to it. For example, U.S. Pat. Nos. 2,627,088 and 2,698,240 teach a primer coating for PET film comprising a terpolymer composition of vinylidene chloride, acrylic ester and itaconic acid. This primer layer is said to have excellent adhesion to the polyester surface and to water or alcohol based photographic gelatin layers substantially coated thereon.
U.S. Pat. No. 3,447,947 teaches a drafting film comprising a PET film base coated with a primer layer which may comprise, inter alia, an organic solvent-soluble mixed isophthalic polyester or polyester-amide or polyester oxazoline. Such layers are said to provide good adhesion to both the polyester base film and to hydrophobic film forming drafting layers applied from organic solvent and containing finely divided toothing agents such as silica, as well as to photographic silver halide emulsion layers and light sensitive diazo layers. However, the application of such coatings by the film manufacturer is not particularly attractive because of the requirement that they be applied as solutions or dispersions in organic solvent. Health and safety factors dictate against the in-plant utilization of organic solvents in coating processes undertaken during film manufacture.
Other primer layers include the thermoset acrylic or methacrylic layers taught in U.S. Pat. No. 3,819,773 which can be applied to the PET film in the plant from aqueous medium. However, these primer layers suffer the disadvantages that they do not provide commercially adequate adhesion for aqueous based coatings such as aqueous printing inks or aqueous matte drafting layers applied directly to them.
Compositions containing certain water dispersible or water soluble copolyesters and copolyester amides have been developed which are taught to be useful as adhesives for various substrates, including metals, papers and polyester film, or as binders or sizing agents for synthetic fibers. Such materials are disclosed in U.S. Pat. Nos. 3,563,942 and 3,779,993. Aqueous dispersions of similar materials are also disclosed in U.S. Pat. No. 3,340,519, which materials comprise copolyester compositions containing the polyester reaction product of an aromatic dicarboxylic acid, an aliphatic dicarboxylic acid, an aromatic dicarboxylic acid containing a metal sulfonate group and polyol components, said composition further containing a water soluble organic compound having a boiling point of 60.degree. C. to 200.degree. C. Such copolyesters are preferably a blend of crystalline and non-crystalline copolyesters. These materials are disclosed to have utility as hot melt adhesives, coatings and binders for plastic fibers or films, including polyester plastics.
More recently, water soluble or dispersible copolyesters of a type similar to those described in U.S. Pat. Nos. 3,563,942 and 3,779,993 above have been disclosed as primer layers for PET film to improve the adhesion to the film of photosensitive layers. For example, U.S. Pat. Nos. 4,252,885 and 4,304,851 teach a priming layer applied to PET film as an aqueous dispersion comprising a polyester having free-functional acid groups derived from the condensation of one or more glycols with one or more tri-functional organic acids. Such primers are disclosed to enhance the adhesion of photograhic layers to PET film. European publication No. 0029620 teaches a PET priming layer useful for gelatin based photosensitive coatings, which primer is based on an aqueous dispersion of the condensation product of ethylene glycol and a mixture of terephthalic acid, isophthalic acid and a salt of sulphoisophthalic acid. U.S. Pat. No. 4,181,528 teaches that the adhesion of hydrophilic photograhic emulsions to PET film is enhanced by the application to the film of a sub-coating comprising an aqueous dispersion of gelatin, a water soluble polyester similar to those discussed above and a polyfunctional aziridine crosslinking agent.
While these and other priming layers or sublayers are effective in enhancing the adhesive qualities of PET film for certain applications, there are limitations. For example, whereas many thermoplastic or thermoset acrylic or methacrylic primers may provide suitable adhesion to photograhic or printing lacquer compositions containing organic solvents, such primers do not provide commercially satisfactory adhesion to many aqueous based coatings or printing inks without the application of a second priming or subbing layer over the first primer. Conversely, many of the water dispersible polyester primers referred to above may provide satisfactory adhesion directly to aqueous or alcohol based systems, but will not provide commercially satisfactory adhesion directly to organic solvent based lacquers or printing inks. Thus the PET film manufacturer often must make available several different types of film each primed with a different primer class or composition to satisfy the differing requirements of his customers. The development of a primer material which would render PET film surfaces receptive to both aqueous and organic solvent coatings or printings applied directly to the primed film surface would be of great significance to the film manufacturer.